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Summary of Significant Accounting Policies - Basis of Consolidation (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounting Policies [Abstract]      
Investments value $ 158.7 $ 145.8 $ 137.9
Unremitted earning from investments $ 103.9 $ 88.9